In hydraulically driven presses or forging machines that can operate with so-called crank-type driving means according to German patent 36 07 737 a piston pump drives a press piston with a pressure multiplying ratio that corresponds to the ratio of the piston areas of the piston pump and the press piston, and with a sinusoidal stroke movement having an amplitude ratio inversely proportional to the cylinder areas. The stroke frequency is determined by the speed of rotation of the drive of the piston pump.
Changing the stroke (stroke volume or stroke length) of the press piston is only possible with a great deal of trouble. However, a typical forging process requires both of these changes. In rough forging or cogging long strokes are used at low frequencies in order to allow sufficient time for transport (feeding) of the forging between the working strokes. To produce the smoothest possible surface at the end of the forging process, in so-called planishing, high stroke frequencies with short strokes are required.
To solve this problem it would seem obvious to provide an adjustable eccentric or crank stroke in the piston pump or the like. The means required to do this are, however, technically so complicated and so unreliable that they have not been adopted in practice.
An alternative described in German patent 23 06 566 is to use two to three switchable stroke lengths. For each press cylinder there is a double piston in which the two piston faces can be connected separately, so that different displacement volumes lead to corresponding different lengths of stroke of the press piston. The stepped pistons needed for this purpose are technically complicated, and in addition only relatively low working frequencies can be used, since the whole mass of the double piston always has to cover the whole stroke.
A disadvantage of all the known devices is that a change in the stroke frequency can only be effected by changing the speed of the drive. Not only does this require a drive equipped with suitable change-speed means, but it also takes quite a long time to change the speed. During the changeover time the machine is not productive.
The starting point of the present invention is that in open die forging, especially bar forging, the operation is divided into two stages with different requirements. The first stage comprises the forming proper: here a large-section ingot is formed into a small-section bar, and as well as the change in shape the internal structure of the material is generally also changed. For this purpose it is necessary to perform both a long working stroke (to obtain a large depth of penetration) and a long feed between each pair of working strokes. The second stage comprises the so-called planishing, i.e. the production of an exactly sized bar that is for example straight and is as smooth as possible. For this purpose relatively short working strokes are required with only a small depth of penetration and a small amount of feed.